In the Art of both residential and commercial window units it is well known to provide tilting window sashes of various constructions. For example, modern single or double-hung windows often include at least one vertically slidable sash which has pivots located at the opposite lateral sides of the sash adjacent to the bottom of the frame stiles and selectively releasable latches located at the opposite lateral sides of the sash adjacent to the top of the respective stiles.
In normal operation, the latches and pivots typically will cooperate with vertical slide channels or tracks in the window jamb to retain the tilting sash for vertical sliding whereby the window is opened and closed. Upon selective release of the upper latches, the sash may be tilted inwardly about the pivots to accommodate cleaning of the exterior glazing surface from within the building.
Various factors including the steadily increasing cost of building maintenance and heightened concern for worker safety have contributed to the demand for such tilting window units. In spite of such demand, however, tilting windows have been subject to certain shortcomings in some instances. For example, modern building codes and architect's specifications, especially for commercial high-rise buildngs, often require the building windows to withstand very large lateral loads without distortion. This has come about in part as a result of increased awareness of the impact a structure like a high-rise office tower can have on such environmental factors as prevailing winds. Indeed, research has clearly demonstrated that the presence of an office tower of given proportions in an air stream can quite readily double the wind velocity of the air passing around the structure, thus resulting in the generation of highly turbulant and forceful air flows and eddies from what would otherwise be merely a gentle breeze. An air flow of such violence can impose loads perpendicular to the exterior glazing surface of up to 50 or 60 pounds per square foot or more on the windows of any building located in the air flow path. In recognition of this phenomenon, window specifications, especially for towers in crowded metropolitan areas, often call for sashes which will withstand 50, 60 or even 100 pounds per square foot of perpendicular force without inward bowing of the window sash stiles or other distortion of the window frame. Any significant inward bowing of the sash stiles would cause loss of seal integrity and result in drafts and water leakage.
Tilt windows in particular have exhibited problems in this regard as the frame of a tilting sash generally cannot be captured by the window jamb along its entire length to be thereby secured against inward bowing without also being incapable of inward tilting. Furthermore, manually operated latches intermediate the ends of the tilting sash stiles, although offering the prospect for sufficient anti-bowing support to satisfy applicable load bearing criteria, have often been objectionable to architects who seek windows with a clean interior finish.